Without limiting the scope of the invention, its background is described in connection with existing Nuclear Magnetic Resonance (NMR) based well logging tools with particular emphasis on wire-line NMR tools. Methods and apparatus that applied NMR to well logging were introduced over fifty years ago. See e.g. U.S. Pat. No. 3,250,986 (McKay). Traditionally NMR has been used to estimate the total pore volume (total porosity) of the source rock by measuring the amount of hydrogen present in the sample. The intensity of the NMR signal is proportional to the amount of hydrogen present in the sample. By calibrating the intensity of the resulting NMR signal to the signal intensity of known fluids, the porosity, permeability and movable fluid characterization of the system can be found.
An assembly of magnetic moments such as those of hydrogen nuclei will act like tiny bar magnets and will align along the direction of an applied magnetic field B0 thus resulting in bulk magnetization of the affected hydrogen protons. The rate at which equilibrium is established in such bulk magnetization upon provision of the static magnetic field is characterized by the parameter T1, the spin-lattice relaxation time or longitudinal magnetic relaxation. T1 is affected by interactions between the resonating protons and their environment (“lattice”) that allow the energy absorbed by the protons during resonance to be dispersed in the lattice. T1 is dependent on molecular dynamics, which is in turn a function of molecular size and thus the viscosity of affected proton containing materials as well as their intermolecular interactions including with pore surfaces in the source rock. Therefore, water and different types of oil have different T1 values. Heavy oils are characterized by T1s in milliseconds while low-viscosity oils may have T1s measured in seconds. These T1 values may be shortened by interactions with pore surfaces thus providing information on pore-size distributions. The T1 value in itself is obviously provides valuable information but a considerable amount of further information can be obtained by manipulating the spins of affected nuclei.
Early NMR tools used the Earth's magnetic field to align the hydrogen protons but such tools required doping of borehole mud fluids with magnetite to eliminate the NMR signal from the borehole itself. See e.g. Bill Kenyon et al., Nuclear Magnetic Resonance Imaging—Technology for the 21st Century, OILFIELD REV., Autumn 1995. In the early 1990's, Numar Corporation developed the first commercial pulsed NMR tool that included permanent, prepolarizing magnets and utilized radio frequency (“RF”) pulses to manipulate the magnetic properties of hydrogen nuclei in formation fluids. See e.g. U.S. Pat. No. 4,717,877, Taicher et al.
Current standard NMR tools are able to provide quantification of important formation characteristics including porosity and saturation, which provide answers to the fluid volumetrics of the formation including the type and viscosity of included fluids and the pore distribution of the formation. Standard modern NMR tools are also able to quantitate the permeability of the formation, which assists in evaluation of the producibility of the formation. Advanced NMR tools are able to quantitate capillary pressure, wettability and surface tension, which also inform on the producibility of the formation and assist in aiding the design of completions. Because they are sourceless, NMR tools are considerably safer than tools that require nuclear radiation to evaluate formation characteristics.
Different formation characteristics are obtained from the relative values of the polarization times, relaxation times and magnetization amplitude. For example, the matrix independent Total Porosity is encoded in the fully polarized NMR amplitude whereas an approximation of permeability of formation can be derived from the rates of magnetization buildup and echo decays, which carry information about fluid interaction with the matrix, fluid type, diffusivity, and viscosity. With multi-frequency tools, basic diffusion based fluid typing can be accomplished. Multi-frequency tools allow measurements from different nuclei including sodium-23 (23Na).
Important goals for any NMR tool are to obtain a relatively high signal-to-noise ratio (“SNR”) and a maximum depth of investigation (“DOI”). The present invention satisfies these needs with a novel and unique geometry between the relative directions of the B0 static magnetic field and the oscillating filed B1.